Known rail-based high speed data transmission includes a mobile system mounted on a rail car and a fixed system positioned trackside or wayside. Each of the mobile system and the fixed system includes a transceiver and an antenna for communication with the other system. Accordingly, as the rail car passes through the section of the track covered by the trackside antenna, data is transmitted between the mobile system and the trackside system. For example, the antenna at each transceiver sends and receives data via linearly polarized radiated fields at a given fixed polarization.
To improve the potential data throughput of known systems, multiple signals are transmitted simultaneously such that the transmitted signals fully share the entire specified frequency band. However, because two or more signals are sharing the same frequency band and wireless link, additional efforts must be taken to ensure that these signals do not interfere with each other.
To that end, in known systems, transmitted signals are linearly polarized. Accordingly, one known system and method to prevent interference includes polarization discrimination. For example, a first linearly polarized signal is typically orthogonal to a second linearly polarized signal. However, such signals require a transceiver that includes transceiving capabilities in both polarizations and an antenna designed and positioned to operate at each desired polarization. Indeed, to realize a two-port dual polarized system, known mobile systems and known trackside or wayside systems each include two individual antennas and mounts, thereby adding to the overall cost of the system.
The antennas typically used in known systems include a driven monopole with directors or a driven dipole end fire array antenna. Undesirably, each of these antennas achieves modest gain in a narrow band while providing varying performance over the band. Furthermore, whichever type of antenna is used, the vertically polarized antenna of the mobile system and the vertically polarized antenna of the trackside or wayside system must be rotated by some amount and mounted on a special platform to achieve the two different polarizations and realize orthogonality between element polarization. This can be physically bulky, mechanically complicated, and introduce additional potential points of failure.
Due to the architecture described above, known systems include the following disadvantages. First, known systems do not have symmetrical vertical and horizontal patterns when measured in free space. Accordingly, when two antennas are rotated and positioned to achieve certain polarizations with respect to their mounting surface, their patterns will not be identical. Second, known systems require a ground plane and perform significantly differently on and off the ground plane or over a non-conductive surface. Third, known systems require a robustly DC grounded driven element to realize high voltage protection. Finally, when multiple antennas are employed, multiple sealing points between the antennas and the rail care are required. This increases installation time, maintenance costs, and the number of potential points of failure.
In some situations, it is desirable to add a third linearly polarized radiating element to the mobile system and to the trackside or wayside system. However, in known systems, a third unique antenna is required to provide a third linearly polarized radiating element, further increasing the height or footprint of the system, necessitating an additional antenna seal, and introducing an antenna pattern performance that does not match the first and second antennas.
In view of the above, there is a need for improved systems.